In a magnetic disk apparatus, a magnetic flux corresponding to a recording current is radiated from a magnetic pole of a magnetic head. The magnetic flux radiated from the recording head magnetizes the magnetic layer of a magnetic disk and forms micro magnets there, whereby information corresponding to the recording current is recorded. When subjected to a magnetic flux corresponding to a new recording current, the very small magnet that has been formed in the magnetic layer of the magnetic disk can change its direction of magnetization. Accordingly, the magnetic disk can be used as a rewritable recording medium.
The recording head is formed of a core constituting a magnetic path and a coil providing the core with a magnetomotive force. A portion of the core is opened to the air, forming a magnetic pole. The recording head, together with a reproduction head, is formed in a head/slider. The head/slider is attached to a head gimbals assembly (HGA) and adapted, when the magnetic disk apparatus performs recording or reproduction operation, to fly over the magnetic disk with a small space held from the surface of the magnetic disk. A magnetic flux corresponding to the recording current is adapted to flow through the recording head, and, when the recording current stops, residual magnetism, of a polarity depending on the direction of the last flow of the magnetic flux, will remain at the end of the magnetic pole opposing the disk surface.
Japanese Patent Publication No. 2003-331402 (“Patent document 1”) discloses a technology to overcome a problem of the residual magnetism, due to a write current passed through the coil of the magnetic head, to cause a distortion of the played back waveform at the time of reproduction. Patent document 1 discloses a technology, in which a magnetic head is moved to a predetermined region of the magnetic disk, such as a CSS region or a gap area when a current is passed through a demagnetizing supply circuit, so that the effect of the recording current to degauss exerted on the recorded data in the magnetic disk may be eliminated.
Japanese Patent Publication No. 2001-307408 (“Patent document 2”) discloses a technology, in which a back electromotive force produced by a spindle motor, continuing to rotate by inertia when the operating supply voltage to a magnetic disk apparatus is lowered, is supplied to a voice coil motor, so that an emergency retracting operation of the magnetic head is performed.
Patent document 1 discloses a technology that prevents occurrence of an error of distorted reproduction waveform when a magnetized signal in the magnetic disk is reproduced while residual magnetism is remains in a recording head. In a magnetic disk apparatus of an in-horizontal-plane magnetic recording system, the magnetic layer is magnetized in a horizontal direction by a leakage magnetic flux flowing through a gap between the upper and the lower magnetic poles of a ring type recording head, and therefore, in order to write information, it is required that a strong magnetic field is generated between the upper magnetic pole and the lower magnetic pole. Accordingly, it seldom occurs that the residual magnetism in the recording head exerts an effect on the information written in the magnetic layer.
On the other hand, in a magnetic disk apparatus of a perpendicular magnetic recording system, the magnetic disk has a magnetic layer and a soft magnetic layer with a high permeability lying thereunder. A write pole and a return pole constituting a monopole type recording head of the perpendicular magnetic recording system is arranged so as to have a larger write gap than the recording head in the in-horizontal-plane magnetic recording system; the magnetic flux radiated from the write pole or return pole passes perpendicularly to the magnetic layer so as to flow through the soft magnetic layer. Since the soft magnetic layer is made of a material having a high permeability, it has a low magnetic reluctance, and, hence, it is liable that a magnetic flux capable of magnetizing the magnetic layer is produced even if residual magnetism remaining in the write pole or the return pole is of a very small amount.
Accordingly, if the recording head of the perpendicular magnetic recording system having the residual magnetism remaining therein is moved over the magnetic disk, such problems arise that information already written in the magnetic disk is rewritten or deteriorated and, thereby, the disk becomes unable to reproduce. In such case, where the supply voltage has abruptly dropped while the magnetic disk apparatus was making recording operation, the retract motion of the head/slider is made before the recording head stops its rotation. If residual magnetism is remained in the recording head at this time, it affects the magnetic layer at the position corresponding to the trajectory of the recording head over the magnetic disk. When the supply voltage has dropped, a power source corresponding to an erasing power supply circuit cannot be obtained by the method as disclosed in Patent document 1.
Regarding the bit information recorded in the magnetic disk, if the number of error bits produced by the effect of residual magnetism in the recording head is small, the user may sometimes be unable to clearly notice the effect caused by residual magnetism, because the magnetic disk apparatus properly corrects the errors by the use of a error correcting code (ECC) or recovers the errors by execution of an error recovery procedure (ERP). However, decreasing the number of the error bits is an important problem in providing a high performance magnetic disk apparatus.